The present invention is concerned with automotive brake control systems of the type wherein each individual wheel is provided with anti-skid brake control means. Such individual wheel anti-skid control arrangements are well known to provide for the shortest possible stopping distance since each wheel brake pressure is predicated upon the available adhesion between the roadway and that particular wheel. However, a disadvantage of systems employing individual wheel anti-skid control is that of disproportionate brake forces arising on different wheels, such as the steering wheels, for example. This is known to cause sudden swerving when the one wheel on one side of the vehicle encounters a slippery (low adhesion) area of the roadway, while the other wheel is running on a dry (high adhesion) surface. The impending wheel skid of the one wheel is sensed by the anti-skid control system which accordingly operates to relieve braking pressure at the one wheel independently of the other wheel to produce the above-mentioned disproportionate brake forces. On vehicles with short wheel bases praticularly, loss of vehicle control may result due to a panic reaction of the operator in attempting to counter steer the vehicle against the direction of yaw brought about by the disproportionate brake forces.
The above discussed problem can be overcome by controlling each wheel according to the dynamic characteristics of the wheel running on the poorer road surface, in other words, the wheel which first experiences an impending skid. This, however, prevents the wheel running on the better road surface from realizing optimum brake torque. Consequently, vehicle stopping distance is compromised in favor of improved steering stability.
In the event the coefficient of friction of the roadway is capable of supporting only minimal braking torque before an impending wheel skid condition arises, the increased stop distance resulting may be such as to create a safety hazard itself, even though improved steering is obtained.
There has been disclosed in U.S. patent application, Ser. No. 457,819, now U.S. Pat. No. 3,918,767, a system of individual wheel anti-skid brake control wherein the brake pressure of a pair of wheels, such as the vehicle steering wheels, is compared and the brake pressure of the wheel having the higher braking pressure is held constant or reduced only when the brake pressure difference between the wheels exceeds a predetermined value. As compared to the above-mentioned system, this pressure difference limiting arrangement permits realization of substantially normal stop distances, while still providing good steering stability.
The comparison of the wheel brake pressures is carried out by a differential pressure switch having an actuating piston subject opposingly to the different wheel brake pressures. The actuating piston is spring biased in a neutral position in which an electrical armature on the actuating piston is positioned between two spaced-apart electrical contacts. The spring bias on the actuating piston establishes a differential pressure setting at which one or the other electrical contacts are engaged by the armature for the purpose of energizing a modulator valve associated with the wheel having the higher braking pressure.
This last-mentioned arrangement has the disadvantage of losing its accuracy over a period of time due to wear on the force transmitting structural parts and fatigue of the bias springs embodied by the differential pressure switch.